Actually does what https://github.com/jquery/jquery/commit/bab8079593913dbc689404aa4e83c46b9b4c9355 promised: passes the jXHR as the third argument of prefilters and transport factories. Comitted for completness even if if backs this out in the end and only land in 1.5.1.

Make sure that init is run before jquery is built (avoiding weird warnings when running 'make jquery' or 'make min'). Additionally only attempt to run JSLint or the minifier if NodeJS is installed (avoiding a weird warning when you first run 'make' or 'make all').

Fixes #5856. Adds document protocol at the beginning of URLs without protocol (thanks go to skrings for the initial pull request). Simplifies cross-domain detection regexp and logic as a consequence. Also took the opportunity to remove an unused variable. Unit test added.